I was trying to find a new motherboard for my MythTV Backend machine that would offer 4 PCI slots but had no luck, so I had to settle for a board with 3 PCI slots and some PCI-E slots.

My next step was purchasing a StarTech PEX1PCI1 adapter card, which takes a PCI card and allows it to be used in a PCI-E 1X slot. The adapter card requires it's own power (see photo) and the HD5500's own mounting bracket had to be removed for the card to fit within a typical full-height expansion slot in the PC's case. Even so, I had to cut away a bit of metal from the case's bracket slot to allow the HD5500's RF socket to clear the mounting hole properly.

The card-on-adapter solution is a bit wobbly in the PCI-E slot, so I also used some plastic shims to hold it in place - you could even do that with rubber bands or cardboard.

The StarTech adapter card is essentially "invisible" to the Linux kernel, so the OS sees the HD5500 just like it does a regularly installed HD5500 and loads the modules properly.